Phytic acid based metallogel and applications thereof

ABSTRACT

The present invention to provide a highly proton conducting metal organic material constituting of phosphate ester based ligand immobilized via gelation with Fe 3+  ion in DMF which is used as conducting electrolyte in PEFMCs.

FIELD OF THE INVENTION

The present invention relates to immobilized phytic acid for fuel cellapplication. Particularly, present invention relates to ferricnitrate-phytic acid (FNPA) metallogel and process for preparationthereof.

BACKGROUND AND PRIOR ART OF THE INVENTION

Metallogels are an important class of supramolecular materials, whoseintrinsic properties stem from the non-covalent interaction between themetallic entity (metal or metal ion), and organic linker (polymer orsmall organic molecules) resulting in a stable extended network withvoluminous immobilization of solvent molecules within it. Most of themetallogelators result from non-covalent interactions, mainly hydrogenbonding apart from metal-ligand coordination. An important property ofmetallogels lies in their ability to conduct protons due to the inherentH-bonding. Therefore, these materials could be utilized as protonconducting solid electrolyte in Proton Exchange Membrane Fuel Cells(PEMFC) as it demands a material with ability to conduct protons at hightemperature (>100° C.) in dry conditions apart from high mechanicalstability. This would essentially perk up the resistance to fuel cellimpurities, improve the electrode kinetics apart from resolving theflooding issues, normally encountered in Nafion® based PEMFCs.

An important alternative for High Temperature Proton Exchange MembraneFuel Cells (HT-PEFMCs) are phosphoric acid doped-Polybenzimidazole (PBI)membranes having high proton conductivity at temperatures up to 200° C.However, one of the problems affecting its performance is the leachingof phosphoric acid at higher levels of doping which limit any furtherimprovement in its conductivity.

In the light of the above, it is evident that any prospective materialthat can be used as a solid electrolyte in a proton exchange fuel cell,needs to satisfy two criteria viz., (1) to separate the anode andcathode components (mainly the reactant gases) (2) to conduct protonsacross it, thereby completing the external electrical circuit and makingthe fuel cell operational for production of electricity.

Therefore, there exists a need for an intrinsically conductingelectrolyte which could not only fasten and selectively transport protonat high temperature (100-200° C.) under anhydrous conditions but alsoeffectively separate the electrode materials and reactant gases for anoptimal overall performance of the PEFMC's. In this perspective,thermally as well as chemically stable metallogels might offer a perfectplatform for immobilizing the proton conducting units via theircoordination to the metal centers. However, till date, there are limitedexamples of such metallogels employed as proton conductors, one being aCuA-Ox xerogel, which exhibits a protonic conductivity of 1.4×10−5 Scm−1 at 65° C. under anhydrous conditions. It is well known that theproton conductivity depends on the number and mobility of chargecarriers (protons). Among the known protogenic molecules, phosphoricacid derived phytic acid (inositol hexakisphosphate) contains 12replaceable protons and is thus capable of easily coordinating tomultivalent ions. Moreover each phytic acid (PA) molecule contains six 6phosphate ester (H₂PO₄) groups, well known for its amphoteric naturethat allows proton conduction without any assistance from externalproton carriers.

Although, phytic acid metallic complexes are known in the art however,these are used to inhibit Polygalacturonase activity in microorganismwhich causes pathogenicity and spoilage of fruits and vegetables duringplant tissue infections (Carbohydrate Polymers, Volume 95, Issue 1, 5June 2013, Pages 167-170).

Article titled “Proton-Conducting Supramolecular Metallogels from theLowest Molecular Weight Assembler Ligand: A Quote for Simplicity” by SSaha et al. published in Chemistry—A European Journal, 2013, 19 (29), pp9562-9568 two novel multifunctional metallogels were readily prepared atroom temperature by simple mixing of stock solutions of Cu¹¹ acetetemonohydrate or Cu¹¹ perchlorate hexahydrate and oxalic acid dihydrate.

Article tided “Proton conductivity enhancement by nanostructural controlof poly(benzimidazole)-phosphoric acid adduct” by J Weber et al.published in Advanced materials, 2008, 20 (13), pp 2595-2598 reportsmesoporous polybenzimidazole doped with phosphoric acid which showsenhanced proton conductivity compared to an equivalent, nonporousmembrane. The introduction of a defined nanostructure into cross-linkedpoly(benzimidazole)/phosphoric acid composites is thus a promisingapproach towards membranes of high temperature stability suitable forfuel cell applications.

CN104022301A discloses a polymer—supported metal-organic frameworkmaterials phytic acid composite membrane preparation method andapplication. The membrane material prepared through the preparationmethod is good in proton conduction property even under low humidity.

Article titled “Enhanced proton conductivity of nafion hybrid membraneunder different humidities by incorporating metal-organic frameworkswith high phytic acid loading” by Z Li et al. published in ACS ApplMater Interfaces, 2014 ;6 (12), pp 9799-9807 reports Nafion/phytic@MILhybrid membranes showed high proton conductivity at different RHs. Inthis study, phytic acid (myo-inositol hexaphosphonic acid) was firstimmobilized by MIL101 via vacuum-assisted impregnation method. Theobtained phytic@MIL101 was then utilized as a novel filler toincorporate into Nafion to fabricate hybrid proton exchange membrane forapplication in PEMFC under different relative humidities (RHs),especially under low RHs.

In the light of the foregoing, there is an unmet need in the art todevelop an intrinsically conducting electrolyte which could not onlyfasten and selectively transport proton at high temperature (100-200°C.) under anhydrous conditions but also effectively separate theelectrode materials and reactant gases, for an optimal overallperformance of the PEFMC's. Accordingly, the inventors of presentinvention had developed a ferric nitrate-phytic acid (FNPA) metallogelfor use as conducting electrolyte in PEFMCs.

OBJECT OF THE INVENTION

The main objective of the present invention to provide a highly protonconducting metal organic material constituting of phosphate ester basedligand immobilized via gelation with Fe³⁺ ion in DMF.

Another objective of present invention is to provide a process for thepreparation of ferric nitrate-phytic acid (FNPA) metallogel byimmobilizing a protogenic ligand (phytic acid) using iron (III) nitratein N, N′-Dimethyformamide (DMF).

Still another objective of present invention is to provide use of aferric nitrate-phytic acid (FNPA) metallogel as conducting electrolytein PEFMCs.

SUMMARY OF THE INVENTION

Accordingly, present invention provides a proton conducting metallogelof a Ferric nitrate-phytic acid, complex.

In an embodiment of the present invention, the proton conductivity ofthe gel is in the range of 8.6×10⁻³ S·cm⁻¹ to 2.4×10⁻² S·cm⁻¹ at 120° C.

In another embodiment, present invention provides a process for thepreparation of proton conducting metallogel and the said processcomprising the steps of:

-   -   mixing Fe (NO₃)₃, 9H₂O (FN) and phytic acid (PA) solution in the        ratio ranging, between 1:1 to 3:1 v/v in solvent to obtain a        solution;    -   aging the solution as obtained so step (i) at temperature in the        range of 80 to 90° C. followed by evaporating at 70-80° C. to        obtain proton conducting metallogel.

In yet another embodiment of the present invention, diameter of thenanospheres is in the range of 10 to 120 nm.

In yet another embodiment of the present invention, wherein the gel isporous or non porous.

In yet another embodiment of the present invention, wherein said gel isused to fabricate membrane electrode assembly (MEA) in fuel cell.

In yet another embodiment of the present invention, wherein said gel isused to fabricate membrane electrode assembly (MEA) in proton exchangemembrane fuel cell (PEMFCs). In yet another embodiment of the presentinvention, wherein open circuit voltages (OCV) of the gel in fabricatedMembrane Electrode Assembly (MEA) is 102 V±0.02 at 120° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows schematic representation of synthesis of FNPA metallogeland Optical micrographs of FNPA metallogel, FNPA xerogel, powdered,xerogel and pelletized FNPA xerogel used for the proton conductionstudies.

FIG. 2: shows the gelation results after 30 min and after aging of 12hrs.

FIG. 3: shows a) HRTEM images evidencing the formation of FNPAmetallogel nanospheres and their eventual aggregation to form a 3Dmetallogel network. b) Frequency and c) Strain dependent studies of FNPAmetallogel (FN: PA=2:1(v/v) in DMF).

FIG. 4: shows Frequency and strain dependent studies of FNPA metallogel(2:1 v/v).

FIG. 5: shows a) Nyquist plots obtained at different temperatures (in °C.); b)

Equivalent circuit determined for the Nyquist plot obtained at 130° C.with circuit model used for data fitting (inset); e) Proton conductivitymeasured during first and second run; d) Schematic representation of MEAmaking using pelletized FNPA Xerogel as solid electrolyte; e) OCVmeasurement; f) Fuel cell polarization plot obtained at 80° C. using dryH₂, Pt, C/FNPA Xerogel/Pt, C, dry O₂ electrochemical cell g)Linearpolarization plot obtained using the electrochemical cell at 80° C.

FIG. 6: depicts TGA plots of FNPA-xerogel before and after protonconductivity measurements, for weight loss.

FIG. 7: shows IR spectra of FNPA xerogel.

FIG. 8: depicts MALDI-TOF MS for FNPA gel.

FIG. 9: a) Strain dependent h) Frequency dependent studies of FNPAmetallogel (2:1 v/v) synthesized at 90° C., c)Strain dependent d)Frequency dependent studies of FNPA metallogel (2:1 and 3:1 v/v )synthesized at RT after 6 days.

FIG. 10: Arrhenius-type plot obtained at different temperatures.

FIG. 11: (a) Pore size distribution of FNPA xerogel (2:1) synthesized at90° C.

FIG. 12: (a) HRTEM of FNPA metallogel synthesized at (b) 90° C. (c) RTafter 6 days.

FIG. 13: High-resolution transmission electron microscopy (HRTEM) imageof FNPA xerogel after water treatment (DI water) for 1 week at roomtemperature i.e. at 20 to 30° C.

FIG. 14: a) Powder X-ray diffraction (PXRD) pattern of FNPA metallogeland xerogel indicating the material's highly amorphous nature. b) TGAplots of FNPA-xerogel before and after proton conductivity measurements.

FIG. 15: TGA profile of FNPA xerogel a); Zoomed view of the TGA profileup to 200° C. b) and isothermal TGA thermograms of FNPA xerogel at threedifferent temperatures (130° C., 150° C. and 160° C.) c). Quantificationof weight loss incurred at each step.

FIG. 16: Concentration-dependent HRTEM analysis of FNPA gel (X=4.65 wt%)

FIG. 17: Equivalent circuit determined for the Nyquist plot obtained at130° C. with circuit model used for data fitting (inset).

FIG. 18: Optical photograph of the fuel cell assembly using fabricatedmembrane electrode assembly (MEA) with palletized FNPA xerogel as solidelectrolyte.

FIG. 19: Lifetime measurement of OCV obtained using the fabricated MEAat 120° C.

FIG. 20: a) Nyquist plot obtained at 120° C. using MEA; b) variation ofMEA membrane conductivity with temperature.

FIG. 21: Plot of power density as a function of temperature indicatingthe influence of pellet thickness (thicknesses of the two pellets usedare 1615 μm and 735 μm).

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, the present invention provides immobilized phytic acidmetallogel for use as conducting electrolyte in PEFMCs. The immobilizedphytic acid metallogel, herein after referred as ‘FNPA metallogel’ or‘FNPA xerogel’ (derived on slow drying) simultaneously in the entirespecification.

Present invention provides a process for preparation of conductingelectrolyte of the invention by immobilizing a protogenic ligand (phyticacid) using transition metal ion in N, N′-Dimethyformamide (DMF) whichresults in a stable metallogel (FNPA).

The transition metals for the purpose of the present invention may beselected from Cr, Co, Fe, Al, Pt, Pd and such like or combinationsthereof. The most preferable transition metal ion is Fe³⁺ ion which isderived from Fe(NO₃)₃, 9H₂O and the metallogel obtained is referred asFNPA (FN=Ferric nitrate nonahydrate; PA=Phytic acid) metallogel.

In further aspect, said gel is used to fabricate membrane electrodeassembly (MEA) in proton exchange membrane fuel cell (PEMFCs).

In still another aspect, the open circuit voltages (OCV) of the gel infabricated Membrane Electrode Assembly (MEA) is 1.02 V±0.02 at 120° C.

The xerogel derived from FNPA obtained upon drying the FNPA metallogel;exhibits a high proton conductivity of 2.4×10⁻² S·cm⁻¹ at 120° C.without assistance of any external agents (moisture, acid or anyheterocycte dopants) which establish that the instant metallogel is aunique supramolecular gel material. The proton conductivity of thexerogel is in the range of 8.6×10⁻³ S·cm⁻¹ to 2.4×10⁻² S·cm⁻¹ at 120° C.The xerogel obtained according to the invention may be porous ornon-porous and the diameter of the nanospheres is in the range of 10 to120 nm.

The invention provides linear feel cell polarization studies to collectthe electrical data using the metallogel of the instant invention. Thesestudies ascertain the completion of the electrical circuit of the fuelcell and thus evidently prove the proton conduction across thepalletized xerogel.

The FNPA metallogel according to the invention is synthesized by asimple one pot procedure at 90° C., wherein, 0.1 mmol of Fe (NO₃)₃, 9H₂O(FN) and 0.1 mmol of Phytic acid (PA) solution in DMF(2:1 v/v) is mixedtogether to form a pale yellow colored solution (FIG. 1). Althoughgelation results within 30 min of heating at 90° C. (as confirmed bytube inversion test), the rheological studies performed on themetallogel indicated it to be of weaker nature. Thus aging of themetallogel is continued for another 12 h in order to further increasethe cross-linking of gelator fibers leading to a metallogel with bettermechanical strength (FIG. 2). A metallogel exhibiting similar propertiescould also be obtained at room temperature when the pale-yellow coloredsolution is left undisturbed for ˜6 days (FIG. 1). The off-white coloredmetallogel obtained is then slowly evaporated at 70-80° C. to obtainFNPA xerogel.

The immobilized phytic acid metallogel according to the invention showseffective entrapment of proton conducting units. The entrapment thusprevents its leaching out during fuel cell operation. As the metallogelis DMF based (B.P=153° C.), the operation temperature is well suited forintermediate temperature Proton exchange Membrane fuel cells (PEMFCs).Since the metallogel is formed due to effective immobilization of Phyticacid via gelation with iron nitrate in DMF, the same helps ineliminating the need for the addition of external proton carriermolecules and further eliminate the problems encountered due to thecarrier leaching during the course of fuel cell operation.

The invention provides characterization studies and proton conductivitystudies of Xerogel produced according to the invention. According tothis aspect, FNPA obtained upon drying the FNPA metallogel is thenpowdered and palletized to obtain xerogel, which is used for protonconductivity studies. The pelletized xerogel exhibits a high protonconductivity of 2.4×10⁻² S·cm⁻¹ at 120° C. without assistance of anyexternal agent (moisture, acid or any heterocycle dopants), therebyestablishes that the xerogel according to the invention is a uniquesupramolecular gel material.

The PXRD pattern of the metallogel as well as xerogel indicated theamorphous nature of the material. Thermogravimetric analysis revealedthat the FNPA xerogel is stable up to 150° C. The HRTEM images of theFNPA metallogel unravels the formation of 20-40 nm sized nanosphereswhich gradually gets fused together into 3D metallogel network. (FIG. 3a) The N2 sorption studies performed on the FNPA xerogel revealed a BET(Brunauer, Emmett and Teller) surface area of 124 m²/g. Metal-ligandcoordination is found to be crucial for gel formation as the gelstrength varied depending on the metal; ligand ratio. The metal-ligandratio according to the invention may be in the range of 2:1 to 1:1. Apreferable metal to ligand ratio of 2:1 in the precursor solution formsgel at a much faster rate at room temperature and showed superiormechanical property, (FIG. 2) compared to the other ratios (FIG. 9).

Further, the viscoelastic gel nature of FNPA is verified by a simpleinversion-tube test (FIG. 1); refer to the two inverted test tubes withthe gel obtained after treatment at RT and 90° C. for 6 days and 30minutes respectively, FNPA metallogel is found to be robust without anyvisual change over a period of 6 months. The viseoelastic nature of thisopaque metallogel is further quantified by oscillatory rheologicalstudies. Dynamic strain sweep (DSS) test (at constant frequency of 1rads-1) shows that the average storage modulus (G′) is one ordermagnitude higher than the loss modulus (G″) within the linearviseoelastic regime (FIG. 3b and 3c ). The storage modulus presentedalmost null dependence on the frequency (i.e., G′w0.047-0.061, tand0.09-0.13, which is characteristic of viseoelastic fibrillar networks,evident from the dynamic frequency sweep measurements (at constantstrain value of 1%). Frequency and strain dependent studies of FNPAmetallogel (2:1 v/v) are also shown in (FIG. 4).

The Fourier transform infrared (FT-IR) spectra were taken in the600-4000 cm⁻¹ region on a Bruker Optics ALPHA-E spectrometer equippedwith universal ZnSe ATR (attenuated total reflection) accessory or usinga Diamond ATR (Golden Gate) (FIG. 7) shows strong peak at 3000 cm⁻¹indicates the P-OH group, and the peaks at 1000 to 1200 indicates theformation of Fe-Phytate Xerogel.

MALDI-TOF study of the FNPA gel as well as xerogel exhibit abundance ofpeaks in low m/z (m/z<1000) region suggesting non-covalent self-assemblyof gelator units (FIG. 8). Moreover, a prominent peak at m/z=781.93(2Fe³⁺+1 Phytic acid+8H⁺) strongly supports presence of the 2:1 complex(Fe³⁺: Phytic acid) as the gelator unit.

The above-mentioned intriguing observations regarding the mechanicalstability as well as rate of formation of FNPA gel based on metal ion(Fe3+) to ligand (phytic acid) ratio hypothesizes that in the smallestgelator unit, only two out of six phosphate ester groups chelates to twoFe3+ cations. The presence of identical groups in the phytic acidmolecules gives all phosphate ester sites an equal propensity to bindwith Fe3+ thereby initially resulting in a non-directionalsupramolecular assembly of the components (Fe-Phytate complex, water,DMF and other possible ingredients, refer FIG. 3). The nanospheres foundin the initial phase of gelation indicate the ready complexation of themultidentate three dimensional phytic acid with Fe3+ in DMF (FIG. 3a ).These nanospheres later aggregate via supramolecular forces (H-bondingor co-ordinate bond) giving rise to nanofibrillar network structurewhich eventually ensnares the whole solvent (DMF) to form monolithic gelstructure. This mechanism further helps to establish the reason behindthe prospective proton conduction. In an average, four out of sixphosphate ester group of a phytic acid molecule remains free afterchelating with two Fe3+ ions. These free phosphoester moieties eithertake part in H-bonding (with another phosphoester group or DMF or water)or remain free. In both of the forms they are capable of conductingprotons efficiently. Moreover, fibrillar structure of the gel networkfurther assists in streamlining the conduction of proton.

The intrinsic proton conducting ability of the FNPA metallogel accordingto the invention is analyzed using two probe A.C impedance measurements(FIG. 5a ). The material is observed to conduct protons over a widerange of temperature (30 to 150° C.) in anhydrous conditions. The sampleis evacuated at 120° C. to eliminate any residual water moleculespresent in the xerogel. In the first set of preliminary experiments,pelletized xerogel is manually pressed between two stainless steelelectrodes and the entire set up is placed inside a N2 flushed,temperature controlled incubator (SH-241, ESPEC Co. Ltd., Japan)connected to the electrochemical work station. The sample is kept ateach temperature for ˜1 h in order to attain thermal equilibrium. Theimpedance measurements are performed at a frequency range of 1 MHz-100Hz with input volt-age amplitude of 10 mV. The Nyquist plots areresolved using the equivalent circuit (Rb+ Cdl/(Rct+WD)+CPEel), whereRb=resistance inherent to the bulk material; Cdl=double layercapacitance; Rct=interfacial charge transfer resistance; WD=WarburgDiffusion; CPE=Constant Phase Element, which accounts for theinhomogeneity and roughness contributions of electrode surface to thetotal impedance (FIG. 5b ).

The plots showed a part of semicircle at high frequency with apronounced tail at low frequency region which could be attributed to thediffusion limitations resulting due to the blocking effects experiencedby protons at the electrodes. This feature thereby excludes thepossibility of electronic conduction in the material which is alsoverified by two probe Direct Current (D.C.) conductivity studies provingthe material to be a good insulator with electrical resistivity of 6×107Ω/cm. High proton conductivity up to 1×10−2 S·cm−1 is obtained at 130°C. On continued heating, the conductivity decreased to 3.6×10−3 S·cm−1at 150° C. due to possible degradation of the material thereafter (asindicated by TGA analysis). However, the sample retained itsconductivity when cooled down to RT from 130° C. and then recycled back(FIG. 5c ). TGA plots of FNPA-xerogel before and after protonconductivity measurements, for weight loss are depicted in FIG. 6.

For direct realization of the material as solid electrolyte andseparating membrane for PEFMC, as a proof of concept, second set ofexperiments are carried out at dry H₂/O₂ fuel cell conditions. D.C.measurements are carried out by fabricating a gas tight 2×2 cm² MEA(Membrane Electrode Assembly) using dry H₂, dry O₂ gases as reactants,Pt-C gas-diffusion electrodes (1 mg of Pt/cm², ELAT, BASF fuel cell) andpelletized FNPA xerogel (cold pressed at 1000 kg N for 2 min) as solidelectrolyte (FIG. 5d ). The electromotive force (emf) measurementscarried out on the MEA showed a starting Open Circuit Voltage (OCV) of0.807 V at 30° C. followed by an increment to a maximum of 1.02V±0.02 at100° C. on thermal activation of the Pt catalyst. Open Circuit Voltage(OCV) remained constant thereafter on further rise in temperature up to120° C. (FIG. 5e ). However, at 130° C. the OCV dropped to 0.85V owingto the fuel cross-over effect. Extrapolating the studies further, animpedance study is simultaneously carried out on the MEA to monitor theentire fuel cell reaction at each temperature (FIG. 5f ).The impedanceresponse included a combination of the responses of the cathode (Pt/C,O₂) and anode half-cells (Pt/C, H₂). The presence of single semi-circleinferred that the time constants (a product of interfacial resistanceand capacitance) of the two half cells are comparable. At 30° C., thelow frequency region of the Nyquist plot clearly showed the presence ofa distinct charge transfer resistance (Rct) due to the catalyst layer,and proton conductivity of 8.6×10⁻³ S·cm⁻¹. With rise in temperature, animprovement in the interfacial charge transfer is observed whichde-escalates the time constant, as evidenced from the disappearance ofsemi-circle. A high proton conductivity value of 2.4×10⁻² S·cm⁻¹ isobtained at 120° C. before the OCV started decreasing due to the fuelcross flow. The activation energy calculated using the Arrheniusequation is found to be 0.19 eV, indicative of a highly efficientGrotthuss pathway for proton conduction, refer FIG. 10.

The invention provides linear fuel cell polarization studies to collectthe electrical data using the metallogel material of the invention.These studies ascertain the completion of the electrical circuit of thefuel cell and thus evidently prove the proton conduction across thepelletized xerogel.

Marking first of such attempt, polarization plots are obtained bydriving the fuel cell reaction using FNPA xerogel as solid electrolyte(pellet thickness=1615 μm) and Pt/C electrodes by passing dry hydrogen(35-50 sccm) and oxygen gases (35-50 sccm) at anode and cathoderespectively. At 0.6V (standard operating potential of PEFMC fuel cell),a power density of 0.55 mW/cm² was achieved at 80° C. (FIG. 5g ). Asimilar polarization test was carried out using a pellet with much lowerthickness (735 μm). A power density of 0.94 mW/cm² at 0.6V verified thecrucial role of pellet thickness in determining the MEA performance. Itwas observed that, for an optimal performance, the pellet should bedenser enough to prevent fuel cross flow, and also thin enough to keepthe cell resistance at its minimal (as Power Density=CurrentDensity×Voltage; V=0.6V). The FNPA metallogel by itself showed a maximumproton conductivity of 2.5×10⁻³ S·cm⁻¹ at 90° C.This shows the evidenteffect of dilution of proton conducting units due to the reducedconnectivity among the phosphoric acid functions owing to the copiousamount of solvent molecules trapped inside the gel network.

EXAMPLES

The following examples are given by way of illustration and thereforeshould not be construed to limit the scope of the invention.

Example 1 Synthesis of Metallogel (FNPA)

The chemicals, Ferric (III) nitrate nonahydrate [Fe (NO3)3.9H2O], Phyticacid solution (50 wt % in water) used in the present invention werepurchased from Sigma Aldrich Chemicals. N, N-Dimethylformamide (DMF) waspurchased from Rankem Chemicals. All starting materials were usedwithout further any further purification.

The FNPA (FN=ferric nitrate nonahydrate; PA=phytic acid) metallogelreported here has been synthesized by a simple one pot procedure at 90°C., wherein 0.1 mmol of Fe (NO₃)₃, 9H₂O (FN) and 0.1 mmol of phytic acid(PA) solution (2:1 v/v) in DMF were mixed together to form a pale yellowcolored solution (S) (FIG. 1). Although the gelation results within 30min of heating at 90° C. (as confirmed by the tube inversion test), therheological studies performed on the metallogel indicated it to be ofweaker nature. Thus, aging of the metallogel was continued for another12 h in order to further increase the cross-linking of gelator fibersleading to a metallogel with better mechanical strength (FIG. S1). Ametallogel exhibiting similar properties could also be obtained at roomtemperature when the pale-yellow colored solution (S) was leftundisturbed for ˜6 days (FIG. 1 and FIG. 2, 9). The off-white coloredmetallogel obtained was then slow evaporated at 70-80° C. The resultingFNPA xerogel was then powdered and pelletized for proton conductivitystudies.

Example 2

i. Rheology, TGA, FTIR and PXRD Experiments

The Rheology experiments were carried out using a force rebalancetransducer equipped TA-ARES rheometer. Couette geometry with cup and bobdiameter of 27, 25 mms and height 38 mm was used for the measurements.The PXRD (Powder X-ray Diffraction patterns) were recorded by means ofPANalytical X′PERT PRO instrument using iron-filtered Cu Kα radiation(λ=1.5406 Å) in the 2θ range of 5-50° with a step size of 0.02° and aramp of 0.3 s/step. The Thermo gravimetric analysis (TGA) experimentswere carried out using a SDT Q600 TG-DTA analyzer from 25-800° C. in N2atmosphere at a heating rate of 10° C. min−1. The Fourier transforminfrared (FT-IR) spectra were taken in the 600-4000 cm−1 region on aBruker Optics ALPHA-E spectrometer equipped with universal ZnSe ATR(attenuated total reflection) accessory or using a Diamond ATR (GoldenGate) as depicted in FIG. 7.

ii. MALDI-TOF MS for FNPA Gel

Matrix assisted laser desorption ionization-time of flight (MALDI-TOF)were performed for FNPA gel using dithranol as matrix. The sampleconcentration was ca.1.0 μM in DMF. Concentration of matrix solution (inTHF) was made to 1 mg/mL and added to the sample solution in 1:1 ratio.The resulting solution was deposited on a stainless steel sample holderand dried under vacuum. The sample was then scanned with N₂ laser(intensity=4500) at a scan rate of 150 shots per spectrum. The sampleswere analyzed under optimized conditions in positive reflectance mode. Apeak at m/z=781.9367 was observed which corresponds to 2Fe³⁺+1 Phyticacid+8H⁺.

Example 3

Proton Conduction experimental details of FNPA Xerogel

i. Determination of Proton Conductivity using Stainless Steel (SS)Electrodes Alternating current (AC) impedance measurements wereperformed to study the proton conducting ability of the metallogel viaquasi-four probe method, ca. 200 mg of FNPA xerogel, obtained on slowdrying of FNPA metallogel at 70-80° C., was pressed using a standard die(13 mm diameter) into pellets of 0.25-0.35 cm thickness (Absolute,Mitutoyo Co. Ltd., Japan with accuracy 0.01 mm) and then evacuated at80° C. under vacuum. The pellet was manually pressed between twostainless steel blocking electrodes. The electrode assembly was thenplaced inside a temperature controllable incubator (SH-241, ESPEC Co,Ltd., Japan) connected to Biologic VPM3 electrochemical work station.The set up was flushed with dry N₂ before the measurement to ensurecomplete dryness. The pellet was the heated slowly from RT to 130° C.The membrane resistance was calculated by fitting the Nyquist plotsobtained at each temperature. The proton conductivity of the pelletizedxerogel was determined using the following relation;

σ=l(R.A)

where σ=proton conductivity (S·cm⁻¹),

-   -   I=pellet thickness (cm),    -   R=resistance of the pellet (Ω) and    -   A=area of the pellet (cm⁻²),    -   ii. Fabrication of Membrane Electrode Assembly (MEA) using        pelletized FNPA xerogel: Standard PEFMC protocol was used for        the MEA fabrication. Initially, FNPA xerogel powder was        pelletized using 2.5 mm diameter die. ca. 800 mg xerogel powder        was used for making each pellet. The electrodes were prepared by        spraying the Pt catalyst ink onto the porous carbon paper        (35CC-SGL with 15% PTFE content). The pellet was then placed in        between the two platinized carbon electrodes [each containing Pt        catalyst (Johnson Matthey)+Valcan carbon support (VX 72)+Nafion        binder (20%)] with Kapton as gasket and cold pressed by applying        1000 KgN pressure for 2 mm. The MEA was then arranged onto        graphite plates using FRT gasket for single cell assembly        (active area=4 cm², Fuel cell Tech). The single cell test        fixture used for fuel cell polarization study consists of        following components:    -   Aluminium end-plates    -   Graphite mono polar plates provided with integrated O-ring        gasket and serpentine gas flow field    -   Cathode loading: 1 mg/cm²; N/C: 0.4; electrode thickness: 320 μm    -   Anode loading: 1 mg/cm²; N/C: 0.4; electrode thickness: 324 μm    -   Gas flow: 0.5 slpm for anode as well as cathode.    -   Operating temperature: RT-120° C.    -   Membrane pellet thickness: 1615 μm    -   Uncompressed MEA thickness: 2259 μm    -   Compressed MEA thickness: 1848 μm    -   % of she compression: 18%    -   Thickness of Gasket used: 714 μm p1 iii. Electromotive force        studies (EMF) of fabricated Membrane Electrode Assembly (MEA)L        For EMF measurements, the cell was fed with pure dry hydrogen        (99.999%) at anode and pure dry O₂ (99.9%) at cathode. The EMF        study showed a starting Open Circuit Voltage (OCV) of 0.807 V at        30° C. On further increasing the temperature, the OCV shoot upto        1.02 V±0.02 at 120° C. and remained constant thereafter. The OCV        was observed to remain stable for the next 5 h, which clearly        reveals the denser nature of the pellet.On further rising the        temperature to 130° C., the OCV immediately dropped to 0.85 V        and thereafter the study was terminated. The sudden decrease in        the OCV could be attributed to the cross flow of the reactant        gases across the pellet membrane.    -   iv. In situ Impedance study on the fabricated Membrane Electrode        Assembly (MEA): The in situ impedance measurements was carried        out via two electrode configuration using Biologic VPM3        electrochemical work station in the frequency range of 1 MHz-100        Hz and 10 mV input voltage amplitude, with O₂ passing cathode        used as working electrode and H₂ passing anode as counter and        reference electrodes. The results were studied using Nyqusit        plots obtained at each temperature (from RT to 120° C.).The        plots were then fit using a PEFMC fuel cell equivalent circuit        and the membrane resistance was calculated determined by the        intercept made on the real axis at the high frequency regime in        the complex impedance plane.    -   v. Direct Current (D.C) linear polarization studies: On        stabilization, linear polarization studies were performed        starting from OCV in 5 mV steps (holding time at each stepwas 1        sec) until the potential decreased to 0.3V.

ADVANTAGES OF THE INVENTION

1. The entrapment of such phosphonate ((H₂PO₄ ⁻) appended ligandeliminates the need for any additional proton carriers. It therebyeradicates the problems of carrier leaching, a limitation of the presentphosphoric acid doped polybenzimidazole (PBI) membranes operating atintermediate temperatures.

2. The Membrane Electrode Assembly (MEA) fabricated using the FNPAxerogel proves to be gas tight giving a maximum OCV of 1.02 V±0.02 at120° C.

3. The in situ impedance measurements performed on the MEA showed thatthe FNPA xerogel is a potential Proton exchange Membrane Fuel Cell(PEFMC) material with a high anhydrous proton conductivity of 2.4×10⁻²S·cm⁻¹ at 120° C.

1. A proton conducting metallogel of a Ferric nitrate-phytic acidcomplex.
 2. The metallogel as claimed in claim 1, wherein the protonconductivity of the gel is in the range of 8.6×10⁻³ S·cm⁻¹ to 2.4×10⁻²S·cm⁻¹ at 120° C.
 3. A process for the preparation of proton conductingmetallogel as claimed in claim 1, wherein said process comprising thesteps of: i. mixing Fe (NO₃)₃, 9H₂O (FN) and phytic acid (PA) solutionin the ratio ranging between 1:1 to 3:1 v/v in solvent to obtain asolution; i. aging the solution as obtained in step (i) at temperaturein the range of 80 to 90° C. followed by evaporating at 70-80° C. toobtain proton conducting metallogel.
 4. The metallogel as claimed inclaim 1, wherein diameter of the nanospheres is in the range of 10 to120 nm.
 5. The metallogel as claimed in claim 1, wherein the gel isporous or non porous.
 6. The metallogel as claimed in claim 1, whereinsaid gel is used to fabricate membrane electrode assembly (MEA) in fuelcell.
 7. The metallogel as claimed in claim 1, wherein said gel is usedto fabricate membrane electrode assembly (MEA) in proton exchangemembrane fuel cell (PEMFCs).
 8. The metallogel as claimed in claim 1,wherein open circuit voltages (OCV) of the gel in fabricated MembraneElectrode Assembly (MEA) is 1.02 V±0.02 at 120° C.